1. Field of the Invention
The present invention relates to an optical amplifying apparatus having a semiconductor laser structure.
2. Related Background Art
A semiconductor optical amplifying apparatus generally comprises a semiconductor laser structure including an active layer and a cladding layer and performs optical amplification of external input light upon reception of a bias current having a threshold value or less. In the field of optical communications, semiconductor optical amplifying apparatuses have been developed as devices for compensating for optical losses occurring in an optical fiber or a connection between optical fibers.
One of the problems posed by use of such a semiconductor optical amplifier in an optical fiber communication system is polarization dependency of an optical amplification factor (i.e., the nature exhibiting different amplification factors for different polarization modes). In general, output light transmitted through an optical fiber does not have a stable polarization state as a function of time. When light having polarization dependency is incident on the optical amplifier described above, the level of output light from the optical amplifier cannot be kept constant but varies as a function of time. A lot of loads such as a requirement of a wide dynamic range are imposed on a receiver system, and the size of a communication system or network is limited, resulting in a decisive drawback.
To the contrary, when a constant polarization plane fiber is used in optical transmission, the above problem can be solved, but a total cost is undesirably increased. In addition, a system using the above fiber does not properly match other systems, thus posing another concomitant problem.
Many attempts have been conventionally made such that an optical amplifier not depending on polarization (i.e., the nature having almost the same amplification factor for different polarization modes) is arranged, or a cause for polarization dependency of the amplification factor is clarified to optimize a laser structure.
Several methods of arranging optical amplifiers not depending on polarization have been proposed. For example, japanese Laid-Open Patent Application No. 1-102983 discloses a method using a polarization rotation unit connected to the input of an optical amplifier. U.S. Pat. No. 4,886,334 discloses a method of splitting input light into two polarization components, inputting these two polarization components into separate optical amplifiers, and combining outputs from the optical amplifiers.
Various examples of the amplifiers not depending on polarization require complicated external optical components and electrical circuits, resulting in low productivity, high cost, and a large size resulting from an optical amplifier module. These problems are inevitably posed when polarization dependency of the amplification factor of the semiconductor optical amplifier is to be indirectly eliminated. Therefore, it is more effective to directly eliminate polarization dependency of the amplification factor of the semiconductor optical amplifier by optimizing a laser structure than using the above methods.
All conventional semiconductor optical amplifiers have been arranged by directly utilizing semiconductor laser structures. A semiconductor laser for performing laser oscillation is generally designed to positively utilize polarization dependency of a gain to stably oscillate a laser beam in a mode of a specific polarization direction. When a laser having the same structure of the above semiconductor laser is used as an optical amplifier, large polarization dependency of the gain occurs.
Factors causing polarization dependency of the gain in a semiconductor laser structure are given as follows:
(1) different confinement coefficients between different polarization modes;
(2) different end face reflectances between different polarization modes; and
(3) different optical gains between different polarization modes.
In order to eliminate factors (1) and (2), it is effective to cause a light wave distribution to come close to a plane wave distribution in a waveguide or near an end face of the amplifier. For this purpose, the most general method is to increase the thickness of an active layer, thereby almost eliminating polarization dependency caused by the factors. Even if the thickness of the active layer itself is not increased, a specific layer structure around the active layer is employed to weaken the light confinement.
The factor (3) is caused by an active layer structure. Polarization dependency does not occur in a bulk active layer. However, when a quantum well structure is used in an active layer, regeneracy is released by a quantum effect to isolate heavy and light holes in a valence band. Light (TE light) having an electric field vector parallel to a quantum well plane has a larger gain than that of light (TM light) having an electric field vector perpendicular to the quantum well plane, thereby causing large polarization dependency.
Polarization dependency occurs when an electron/hole confinement direction is limited to one direction, i.e., when the quantum well structure is a one-dimensional quantum well structure for one-dimensionally confining carriers. Polarization dependency is eliminated in a two-dimensional quantum well structure (e.g., a quantum line structure) for confining electrons and holes in two directions.
When only polarization dependency caused by different optical gains as the factor (3) is taken into consideration, the active layer is arranged to have a bulk structure or a two-dimensional quantum well structure (e.g., a quantum line structure). Therefore, no problem is posed when the one-dimensional quantum well structure is not employed.
The one-dimensional quantum well structure is generally used in a semiconductor laser structure and is more advantageous in design and fabrication than a bulk structure. In addition, it is known that the one-dimensional quantum well structure as an optical amplifier can improve amplification characteristics. Effectiveness of the one-dimensional quantum well structure cannot be neglected.
Although a technique for fabricating a thin quantum line has been developed, two-dimensional confinement of carriers must be equally performed to obtain perfect nondependency on polarization. For this purpose, it is assumed that processing with a precision of several .ANG. to several hundreds of .ANG. must be achieved in a lateral direction perpendicular to a stacking direction. It is, however, very difficult to form such a thin quantum line.
When a conventional semiconductor optical amplifying apparatus structure is used without any modifications and is formed into a module, this module becomes inappropriate as described above. Even if the conventional apparatus structure is used in an optical communication system or network, quality of optical communication is degraded, and the system configuration becomes complicated.